The present invention relates to a method for producing spinnable polyamides of the so-called nylon type by a conversion and polycondensation of dinitriles with aliphatic .alpha.,.omega.-diamines and excess water in the presence of catalysts of an inorganic or organic nature in catalytic quantities.
The method disclosed herein is utilized for the continuous production of polyhexamethylene adipamide, Nylon-6.6, from adiponitrile and hexamethylene diamine in the presence of ammonia-containing water and small quantities of catalysts, preferably oxygen-containing compounds of phosphorus which exert an accelerating influence on the total kinetics of the process, thereby facilitating a higher degree of polymerization and/or reduction in the time required for said polymerization.
Up to now, adipic acid, that could be produced only from aromatics, has been used to produce polyamides. A separate saponification of adiponitrile into adipic acid is bothersome, does not proceed well quantitatively, and is therefore not considered practical. At present, adiponitrile can be produced considerably more economically electrochemically from propylene or acrylonitrile, as well as catalytically by the conversion of butadiene and hydrogen cyanide on a large scale and with high purity.
At the present time, adipic acid is shipped by costly transport means in solid form, and hexamethylene diamine in a liquified state with a small amount of water in separate containers; or a neutralized solution of adipic acid and hexamethylene diamine, each approximately in a 60% concentration, is shipped in heated tank cars. Aside from the difficulties in connection with longer distances, the co-shipping of approximately 40% water is costly.
The shipping of solid salt comprising equimolar quantities of adipic acid and hexamethylene diamine, the so-called AH-salt, does not eliminate the above difficulties. This is due to the fact that the AH-salt must be obtained in a costly manner with respect to energy by evaporation or precipitation from water or methanol. For further processing, again, a 60% AH-salt solution, increased by the water from the reaction, must be evaporated during the polycondensation.
Literature on the reaction between adiponitrile, hexamethylene diamine and water in excess does not mention an efficient technical process for the reaction. Thus, in Greenewalt, U.S. Pat. No. 2,245,129, a total reaction time of more than 20 hours is required for a conversion as in example II therein. Also, in Onsager, U.S. Pat. No. 3,847,876, all disadvantages, which would be alleviated by an economical, continuous working method, are not eliminated, especially because Onsager calls for a minimum content of 1% of free ammonia in the reaction product. Thus, in examples 1-10 therein, which led to spinnable products, total reaction times of more than 8 hours were required. By a recycling, as in example 3 therein, a residence time spectrum was brought about and reached, which was unfavorable and undesirable for polyhexamethylene adipamide.
From experience it has been known that, based on the thermal instability of polyhexamethylene adipamide, short residence time and good residence time spectra are desirable, in all phases of the method, and which become more critical with an increasing degree of polycondensation and with increasing temperatures. It is with this background that the present invention was developed.